I found it frustrating when I recently posted my Google Sites page to Facebook and saw a blank image in my Facebook post.
I read up on the issue, and apparently Google Sites does not properly encode the “<meta property=”og:image”….” tag, which causes problems.
I finally found a hack that barely gets me by. I went into the settings of my Google Sites page and edited the “logo” for the site. Hover in the top left corner and you’ll see an “edit logo” button. Set that image to be the image you wish to see in your Facebook post.
Then use this tool to scrape your Google Sites page and make sure Facebook is displaying what you want to see. It’s called the Facebook Sharing Debugger. https://developers.facebook.com/tools/debug/sharing/
If everything looks good in the Debugger Tool, then go back to your Facebook post (if you’ve already posted it) and edit it. Replace the old URL in your post with the the updated URL (which might be the same, but go ahead and put it in again anyway).
After you hit the post button you should see the correct image along with your post. What a relief it will be to not see that blank greyed out image box again!